Conventionally, Patent Document 1 (JP 333176582 corresponding to U.S. Pat. No. 5,526,650) discloses regarding a heat pump cycle (vapor-compressing refrigerant cycle) used for a vehicle air conditioner. The heat pump cycle includes an interior condenser (heating heat exchanger) in which high-temperature and high-pressure refrigerant discharged from a compressor radiates heat via heat exchange with air, an exterior heat exchanger in which the refrigerant having passed through the interior condenser exchanges heat with outside air, and an interior evaporator (cooling heat exchanger) in which the refrigerant flowing out of the exterior heat exchanger evaporates via heat exchange with air. The interior condenser is arranged downstream of the interior evaporator in a flow direction of air to be blown into a vehicle compartment which is a space to be air-conditioned. The blown air having been cooled and dehumidified by a heat absorption effect due to evaporation of refrigerant in the interior evaporator is reheated by utilizing the refrigerant discharged from the compressor as a heat source in the interior condenser, so that dehumidifying-heating of the vehicle compartment is performed.
The heat pump cycle further includes a high-pressure side expansion device which decompresses refrigerant flowing out of the interior condenser and discharges the decompressed refrigerant to the exterior heat exchanger, and a low-pressure side expansion device which decompresses refrigerant flowing out of the exterior heat exchanger and discharges the decompressed refrigerant to the interior evaporator, so as to improve air heating capacity of the interior condenser.
Generally, in such a heat pump cycle, a refrigerant evaporation temperature in the interior evaporator is required to be maintained equal to or higher than a predetermined temperature (e.g. 1° C.), which is higher than a frost-formation temperature (i.e., 0° C.), in order to prevent frost formation on the interior evaporator.
Hence, in the heat pump cycle described in Patent Document 1, when a valve open degree of the high-pressure side expansion device becomes smallest, and when a valve open degree of the low-pressure side expansion device becomes largest, both the exterior heat exchanger and the interior evaporator are used as evaporators in which refrigerant evaporates approximately at the same pressure. In this case, refrigerant evaporation temperatures in the exterior heat exchanger and the interior evaporator are kept at predetermined temperatures.
As a result, heat absorption amounts of refrigerant in both the exterior heat exchanger and the interior evaporator cannot be increased any more. Therefore, if the interior condenser is further required to heat air, it may be impossible to increase the air heating capacity of the interior condenser sufficiently.